Piston for cylinder and piston unit of disc brake

ABSTRACT

A piston ( 30 ) for a cylinder and piston unit ( 24 ) of a disc brake ( 10 ), with a novel capability of damping the vibrations induced by a braking action, comprising a body ( 32 ) and a head ( 34 ) designed to come into contact with a pad ( 20 ) to act on a braking strip ( 13 ) of a disc ( 12 ), a damping element provided between the head ( 34 ) and the body ( 32 ) of the piston ( 30 ), in which the head ( 34 ) and the body ( 32 ) comprise, respectively, a stem ( 62 ) and a recess ( 41 ) for the stem ( 62 ), forming a gap ( 64 ) for the damping element.

The present invention relates to a piston for a cylinder and piston unitof a disc brake, particularly, but not exclusively, designed for discbrakes for application to motor vehicles.

A known piston for a cylinder and piston unit, comprising a body and ahead designed to come into contact with a pad to act on a braking stripof a disc.

It is also known that pistons of the type indicated above press the padsagainst annular braking strips of the disc to generate a requisitebraking action, for stopping a vehicle, for example. When pressedagainst the braking strip of the disc during braking, the pads arestressed by the disc with a force directed tangentially to the disc,this force being opposed to achieve the desired braking effect. For thisreason, the pads come to bear on supports provided in the caliper, whichreact against the tangential forces.

As is also known, vibrations may occur in the disc brake during thisbraking action. These vibrations of the disc brake are manifested in theform of screeching which is in itself annoying.

The problem to which the present invention relates is that of proposinga piston for a cylinder and piston unit of a disc brake which hasstructural and functional characteristics such that the aforesaiddisadvantages cited with reference to the prior art are overcome.

This problem is resolved with a piston for a cylinder and piston unit ofthe type specified above, characterized in that it comprises a dampingelement between the head and the body of the piston.

Further characteristics and the advantages of the piston for a cylinderand piston unit of a disc brake according to the invention will be madeclear by the following description of a preferred embodiment of theinvention, provided for guidance and without limiting intent withreference to the attached figures, in which:

FIG. 1 shows a schematic view of a disc brake from above and in partialsection;

FIG. 2 shows a view in partial section of a piston for a cylinder andpiston unit of the disc brake in FIG. 1;

FIG. 3 shows a sectional view of a piston for a cylinder and piston unitof a disc brake according to a second embodiment of the invention;

FIG. 4 shows a sectional view of a piston for a cylinder and piston unitof a disc brake according to a third embodiment of the invention, and

FIG. 5 shows a sectional view of a piston for a cylinder and piston unitof a disc brake according to a further embodiment of the invention.

With reference to the aforesaid figures, the number 10 represents thewhole of a disc brake, particularly suitable for a high-performancevehicle. The disc brake comprises a disc 12, having an annular brakingstrip 13 with the axis X—X.

The disc brake 10 also comprises a caliper housing 14, which extends onboth sides of the braking strip 13 of the disc 12.

During the forward running of the vehicle, the braking strip 13 movesinside the caliper housing 14 in a direction V1. During reverse running,however, the strip 13 moves in a direction indicated by V2, opposite toV1.

The housing of the caliper 14 is formed by two half-housings 15 and 16,produced by casting from a suitable aluminum alloy, the half-housingsbeing juxtaposed on a plane perpendicular to the axis X—X, and beingfixed to each other by means of screws 17.

Sockets 18 for receiving the pads 19,20, located on opposite sides ofthe strip 13, are formed in the housing 14.

The pads 19, 20 comprise steel plates 21, 22 respectively, and frictionlinings 21 a and 22 a respectively, made from a material having a highcoefficient of friction, the linings 21 a and 22 a being supportedstably by the plates 21, 22, and being in direct contact with the facesof the braking strip 13 during the braking action.

The pads 19 and 20 are prevented from falling out by conventionalretaining means 23, comprising pins extending through slotted holesformed in the plates 21, 22.

Two opposing pairs of cylinder and piston units 24, mounted in thehalf-housings 15, 16, are active during the braking action to press thepads 19, 20 against the corresponding faces of the braking strip 13, aswill be described in greater detail below.

As a result of a braking action in forward running, the braking strip 13exerts a force directed tangentially to the disc, as shown by the arrowV1, on the pads 19 and 20. The disc brake 10 comprises a support 25 (26)in the caliper housing 14 to retain the pad 19 (20).

To retain the pad 19 (20) following a braking action in reverse running,which exerts a force in the direction shown by the arrow V2, the discbrake 10 comprises a further support 27 (28).

The aforesaid braking actions are generated by supplying working fluidto the cylinder and piston unit 24 by means of a conventional hydraulicbrake circuit.

The shapes and diameters of the cylinder and piston units 24 areidentical or proportional to each other, and therefore only one of theseunits is described in detail below.

A piston 30 for a cylinder and piston unit 24 comprises a body 32, oroperating part of the piston, interacting with a head 34, or thrust partof the piston, designed to come into contact with the plate 22 of thepad 20. The body 32 is fitted in a fluid-tight way in a cylinder 36,formed in the half-housing 16 of the caliper 15, forming a thrustchamber 38. The body 32 of the piston 30, guided by the cylinder 36, canslide along the axis Y—Y of the latter under the action of the workingfluid present in the thrust chamber 38. Tightness with respect to theworking fluid is provided by sealing means 40, such as a ring seal,housed in a channel of the cylinder wall and acting on the body 32 ofthe piston (FIG. 1).

In one embodiment of the invention, the body 32 of the piston iscup-shaped. Thus a body 32 of the piston having a recess 41, or in otherwords an inner chamber, facing the pad 19, is provided. The cup-shapedbody 32 has an annular lateral wall 42 provided with an outer surface44, designed to slide in the cylinder 36, and an inner surface 46,laterally delimiting the recess 41, together with a base 48, arranged sothat it closes the annular lateral wall 42 at its end opposite the pad19. The base 48 has an inner surface 50, delimiting the recess 41, andan outer surface 32, designed to receive the thrust action of theworking fluid present in the thrust chamber 38. In a particularembodiment of the invention, the cup-shaped body 32 is cylindrical andhas a recess 41 whose depth is approximately equal to the diameter ofthe recess 41 (FIG. 2).

In the inner surface 50 of the base 48 there is provided a bowl-shapedseat 54, for example a seat 54 in the form of a spherical bowl-shapedhollow. An annular channel or seat 56 is formed in the inner surface 46of the lateral 42, and is preferably located near the free end 58 of thelateral wall 42, at a predetermined distance (D) from the base 48. Theend 58 has an annular flaring or channel 60 at the inner edge of thecup-shaped body 32, designed to widen the entry of the recess 41 of thecup-shaped body 32. Said body 32 is preferably made from metallicmaterial, for example aluminum.

The head 34 of the piston comprises a stem 62 designed to be housed inthe recess 41 of the cup-shaped body 32. The stem 62 is housed in therecess 41 of the cup-shaped body 32 in such a way as to form a gap 64between its lateral surface 66 and the inner surface 46 of the lateralwall 42 of the cup-shaped body 32. For example, the stem 62 of the head34 is cylindrical and has an external diameter equal to half thediameter of the cylindrical recess 41, and the gap 64 has a width equalto half the diameter of the stem 62 of the head 34.

The stem 62 is in contact with the cup-shaped body 32. The contactbetween the head and the cup-shaped body 32 is provided by making thefree end 63 of the stem bear on the base 48 of the cup-shaped body 32.For example, the free end 63 of the stem is shaped in the form of a dome68, and in particular is shaped in the form of a spherical dome 68. Thespherical dome 68 is housed in the bowl-shaped seat 54 provided on thebase 48 of the cup-shaped body 32, and preferably the end 63 of the stemand the bowl-shaped seat 54 of the base 48 have conjugate surfaces. Theend 63 and said bowl-shaped seat 54 form a joint, and in particular aball joint, between the body 32 and the bead 34 of the piston. The jointallows the head 34 to be made to oscillate with respect to thecup-shaped body 32. In particular, because of the presence of the joint,the axis Y′—Y′ of the stem 62, and consequently of the head 34, can beorientated in a different direction from that defined by the axis Y—Y ofthe cup-shaped body 32.

The stem is also provided with annular projections 70, 72 which delimita free portion of the stem. The annular projections 70, 72 have endsurfaces aligned with each other in such a way as to form a support anda guide. Retaining or snap-fitting means are provided at a predetermineddistance (D) from the free end 63 of the stem, and preferably at adistance (D) equal to the distance (D) from the base at which the seat56 is formed in the wall of the cup-shaped body 32. For example, acircumferential channel 74 is provided, to form a seat capable ofhousing an elastic wire ring 76. In operating conditions, the elasticring 76 partially projects from the annular channel 74.

In one embodiment of the invention, the head 34 of the piston 30 ismushroom-shaped and its top consists of a dish 78. The dish 78 isprovided externally with a thrust surface 80 designed to interact withthe plate 22 of the pad 20. A plurality of channels 82, commonly knownas scores, is formed in the thrust surface 80 and divides thrust surface80 into portions or areas 84 separated from each other and designed tobear on the plate 22. For example, the scores are formed by parallelchannels 82, or, alternatively, by concentric circumferential channels.The head 34 is internally hollow. For example, a blind hole 86 passingthrough a considerable part of the head 34 is provided, coaxial with theaxis Y′—Y′ of the head 34. For example, the blind hole 86 extends withinthe stem 62 to a point near the spherical bowl 68. The blind hole 86 hasportions of different diameters, decreasing progressively from the dishportion 78 of the head 34 to the stem portion of the head 34. Theaforesaid shaping of the cavity of the head 34 makes it possible to havea large surface of the cavity without reducing the resistance of thehead 34 to the thrust imparted by the cup-shaped body 32 and transmittedto the pad 20. A plurality of radially positioned through holes 88 isprovided in the head 34, near the dish 78. For example, the plurality ofthrough holes 88 puts the exterior of the head 34 into communicationwith the internal cavity 86. Preferably, the through holes 88 areequally spaced from each other.

The head 34 is made from metallic material, for example from titanium orpreferably steel.

Advantageously, a damping element is housed in the gap present betweenthe stem 62 of the head 34 and the lateral wall 42 of the cup-shapedbody 32. The term “damping element” denotes an element capable ofdamping the vibrations induced in the head 34 of the piston 30 by abraking action. For example, a ring 90 of damping material, such as asynthetic material or a natural rubber, or a material generally knownfor making anti-vibration or silent block supports, is housed in the gap64. In one embodiment of the invention, a ring of EPDM or,alternatively, of synthetic silicone-based material, such as thematerial commonly known as VITON, is housed in the gap 64.

More advantageously, the damping ring 90 is associated with an innerlining 92, on the surface in contact with the stem, and an outer lining94, on the surface in contact with the cup-shaped body 32. For example,the linings 92, 94 are concentric metal bushes on whose opposingsurfaces the damping ring 90 is fixed. The damping ring 90 isadvantageously housed in the gap by force fitting. In particular, theouter lining 94 is housed in the annular seat 56 provided in the lateralwall 42 of the cup-shaped body 32 and the inner lining 92 is supportedby the annular projections 70, 72 of the stem 62. The inner lining 92 istapered from the central portion to the ends or, in other words, hasflarings 96, 98 on the edges fading the stem 62. When in the operatingposition, the damping ring 90 bears on the elastic wire ring 76 whichpartially projects from the circumferential channel 74 of the stem 62.

Advantageously, the damping ring 90 has a height equal to the width ordiameter of the stem 62 and a width at least equal to half the width ofthe stem 62.

The assembly and operation of a piston for a cylinder and piston unit ofa disc brake according to the present invention is described below.

For the assembly of the piston 30, the cup-shaped body 32 and thedamping ring 90 are first fitted together. The damping ring 90 isinserted into the recess 41 of the cup-shaped body 32, the outer lining94 being forced into the seat 56 provided in the lateral wall 42. Thisoperation is facilitated by the provision of the flaring 60 at the entryof the recess 41 of the cup-shaped body 32. When the damping ring 90 hasbeen inserted, the head 34 of the piston 30 is fitted. The stem 62 isinserted into the inner lining 92 until the end 63 shaped in the form ofa spherical dome 68 is brought into contact with the bowl-shaped seat 54in the base 48 of the cup-shaped body 32. In other words, the annularprojections 70, 72 slide on the inner lining 92 until the end 63 of thestem 62 shaped in the form of a spherical dome 68 is settled in the seat54 located on the base 48 of the cup-shaped body 32. This operation isfacilitated by the provision of the flarings 96, 98 of the inner lining92 which allows the lining 92 to be slid on the annular projections 70,72 of the stem 62. The flaring 96 provided in the inner lining 92 alsoallows the elastic wire ring 76 to be moved completely within thecircumferential channel 74 during the sliding of the stem 62 in thedamping ring 90. When the stem has been brought into the operatingposition, the damping ring 90 is positioned beyond the circumferentialchannel 74, as a result of the provision of the circumferential channel74 at a distance (D) from the end 63 of the stem 62 equal to thedistance (D) of the seat 56–for the damping ring 90 from the base 48, insuch a way as to allow the elastic wire ring 76 to emerge from thechannel 74 to form a stop and bearing element which prevents the stem 62from falling out by snap-locking the stem 62 in the operating position.The piston 30 which is now assembled can then be housed in aconventional way in the cylinder 36 provided in the caliper housing 14so that it can be pushed by the working fluid provided in the thrustchamber 38 against the pad 20.

During a braking action, the piston 30, pushed by the brake fluid,presses the pad 20 against the braking strip 13 of the disc 12, thusgenerating a braking action from which vibrations may arise. Thesevibrations from the pad are partially transmitted through the thrustsurface of the dish to the head of the piston. As the head moves orvibrates relative to the body of the piston, it deforms the damping ringwhich damps the vibrations.

In other words, the energy present in the vibration transmitted from thepad to the piston head is partially dissipated by the damping ring,which, by damping, or in other words attenuating, the vibration of thepad and the vibration induced in the disc by the braking action,eliminates annoying screeches. The dimensions of the damping ring andits material are selected in such a way as to damp a vibration inducedby a braking action and having a given frequency distribution.

As may be appreciated from the above description, the piston for acylinder and piston unit of a disc brake according to the presentinvention makes it possible to meet the aforesaid requirement to dampthe vibration induced by the braking action, and consequently toeliminate the annoying screeches produced during the braking action.

In particular, owing to the provision of a damping element interposedbetween the body and the head of the piston, it is possible to damp thevibrations produced during the braking action and transmitted to thecaliper housing through the cylinder and piston unit.

Moreover, the mushroom-shaped piston head provided with a dish-shapedtop enables the pressure exerted on the pad to be controlled precisely.In other words, the dish makes it possible to have a larger thrustsurface or bearing area of the piston, enabling the pressure on the padto be reduced and allowing the pad to be pressed uniformly against thebraking strip of the disc.

Owing to the provision of a piston comprising a head and a cup-shapedbody, and the provision of a hollow head, through ventilation holes, andthe scores of the thrust surface of the dish, it is possible to obtainexcellent thermal insulation. Thus the heat produced during the brakingaction and transmitted to the cylinder and piston unit is largelydissipated by the extended heat exchange surface of the head before itreaches the body of the piston and the fluid contained in the thrustchamber.

Owing to the limited contact provided between the stem of the head andbody of the piston, it is also possible to obtain further thermalinsulation of the piston body.

Since the contact between the head and body is provided by a sphericaljoint, the piston is made to exert its thrust action on the pad in anoptimal way, even if there is a deformation of the caliper housing dueto the braking action. The ball joint also makes it possible to have anoptimal thrust, even if there is non-uniform wear of the frictionlining, by making the pad position itself with the plate on a planewhich is oblique with respect to the annular braking strip. This isbecause, in such a case, the piston head will be inclined with respectto the axis of the piston body, enabling the thrust surface to bearfully against the plate of the pad.

A further advantage of the invention lies in its novel structuralsimplicity, which enables it to be produced at a very low cost.

With particular advantage, the free end 63 of the stem 62 is shaped inthe form of a dome 68 having a radius of a dimension sufficient todispose a rotational centre or swing centre of the head 34 in proximityof the head end faced to the pad. For example, as shown in FIG. 2, thehead swing centre is substantially disposed on the base of the dish 78.Due to this head swing centre arrangement, even small vibrationamplitude of the dish are magnified by the stem, generating magnifiedvibration amplitude and magnified deformation of the dumping element,enabling an efficient vibration attenuation transferred to thecup-shaped body.

Clearly, variations and/or additions may be made to the inventiondescribed and illustrated above.

As an alternative to what is shown in FIG. 2, the head 34 may behorn-shaped and the cavity 100 formed in it may be funnel-shaped (FIG.3).

To increase the surface of the head and consequently to increase theheat exchange surface, it is possible to provide fins 102 extendingradially from the head 34 in the proximity of the dish or actually onthe dish (FIGS. 3, 4 and 5).

As an alternative to a cup-shaped body 32, it is possible to provide abody 32 comprising a sleeve 104 closed by a base plate 106 (FIG. 4).

As an alternative to the end of the stem 62 shaped in the form of aspherical dome 68, it is possible to provide a threaded member 108provided with a spherical head 110 (FIGS. 4 and 5).

A person skilled in the art may, in order to meet contingent andspecific requirements, make numerous modifications and adaptations tothe preferred embodiment of the piston for a cylinder and piston unit ofa disc brake as described above, or replace elements with otherfunctionally equivalent ones, without departing from the scope of thefollowing claims.

1. Piston for a cylinder and piston unit of a disc brake, said pistoncomprising: a body and a head, configured to move within said cylinder,designed to come into contact with a pad to act on a braking strip of adisc, said head having a stem and said body having a recess for saidstem, wherein a gap for a ring damping element is disposed between saidhead and said body of said piston, wherein said ring is associated withlinings on surfaces in contact with said stem and said body of saidpiston such that said ring guides and supports said stem by means ofannular projections provided on said stem.
 2. Piston according to claim1, characterized in that the ring has a width equal to half that of thestem and a height equal to that of the stem.
 3. Piston according toclaim 1, characterized in that the said ring is made from syntheticmaterial or natural rubber.
 4. Piston according to claim 1,characterized in that said ring is made from EPDM or from silicone-basedsynthetic material.
 5. Piston according to claim 1, characterized inthat said ring is force-fitted into the gap.
 6. Piston according toclaim 1, characterized in that the ring is housed in an annular seat ofthe body of the piston.
 7. Piston according to claim 1, characterized inthat edges of one of said linings of the ring are reamed in such a wayas to form a funnel shape for receiving the stem in the ring.
 8. Pistonaccording to claim 1, characterized in that said stem further comprisesa contact between the stem and the body of the piston.
 9. Pistonaccording to claim 8, characterized in that said contact between thestem and body of the piston is provided by means of a ball joint. 10.Piston according to claim 1, characterized in that the body of thepiston is made in the shape of a cup whose cavity is designed to receivethe stem.
 11. Piston according to claim 10, characterized in that saidbody of the piston in the shape of a cup further comprises a base inwhich is formed a seat designed to receive an end of the stem shaped inthe form of a spherical dome.
 12. Piston according to claim 11,characterized in that the body of the piston is a sleeve closed by abase plate.
 13. Piston according to claim 11, characterized in that athreaded member with a spherical head is connected to the end of thestem.
 14. Piston according to claim 1, characterized in that the end ofthe stem is mushroom-shaped and its top is comprised of a dish. 15.Piston according to claim 14, characterized in that the dish has anexternal thrust surface designed to interact with the pad.
 16. Pistonaccording to claim 15, characterized in that the external thrust surfaceis scored.
 17. Piston according to claim 1, characterized in that thehead is internally hollow.
 18. Piston according to claim 17,characterized in that the hollow head is provided with a plurality ofradial through holes.
 19. Piston according to claim 14, characterized inthat the head has radially extending fins.
 20. A piston according toclaim 1, wherein said piston of said cylinder and piston unit is withina caliper housing of a disk brake.